氧同位素示踪夏季北冰洋(62.3°~74.7°N)大气硝酸盐形成途径的研究

Using oxygen isotopes to trace the formation pathways of atmospheric nitrate over summer Arctic Ocean(62.3°~74.7°N)

大气硝酸盐(包括颗粒态硝酸盐和气态硝酸)是一种重要的含氮物质,在北冰洋氮的生物地球化学循环中起着重要作用。然而,目前关于北冰洋上大气硝酸盐形成机制的研究较少,限制了对该地区氮氧化物(NO_(X))到硝酸盐相关大气化学过程的理解。作为2012年夏季中国第五次北极科学考察的内容之一,本研究采集了科考航线上的大气气溶胶样品,并对北冰洋航段(62.3°~74.7°N)上样品中硝酸盐的氮氧同位素(δ^(15)N,δ^(17)O和δ^(18)O)进行了分析以研究该区域大气硝酸盐的形成过程。观测到的Δ^(17)O(NO^(–)_(3))变化范围为21.7‰~28.8‰,均值为(25.4±2.7)‰;δ^(15)N(NO^(–)_(3))变化范为是-7.5‰~0.8‰,均值为(-4.2±3.0)‰。整体上,Δ^(17)O(NO^(–)_(3))与采样纬度呈现相反的变化趋势,与夜间时长和O_(3)浓度呈现相似的变化趋势;δ^(15)N(NO^(–)_(3))与气温呈现相反的变化趋势。基于化学动力学的分析表明,Δ^(17)O(NO^(–)_(3))的变化可能主要反映的是NO_(2)+OH、N_(2)O_(5)+H_(2)O(aq)、NO_(3)+HC/DMS、NO_(3)+H_(2)O(aq)、XNO_(3)+H_(2)O(aq)(X=Br、Cl、I)等硝酸盐生成途径的变化。基于Δ^(17)O(NO^(–)_(3))的计算表明:低Δ^(17)O(NO^(–)_(3))样品[Δ^(17)O(NO^(–)_(3))=21.7‰~24.5‰,66.2°~74.7°N]的主导反应为NO_(2)+OH,其对硝酸盐的可能平均贡献是68%~81%;对于高Δ^(17)O(NO^(–)_(3))样品[Δ^(17)O(NO^(–)_(3))=27.5‰~28.8‰,62.3°~69.9°N],NO_(3)+HC/DMS、NO_(3)+H_(2)O(aq)和XNO_(3)+H_(2)O(aq)三者一起的贡献最高,可达35%~50%。结合BrO柱浓度的分析表明,XNO_(3)+H_(2)O(aq)反应对高Δ^(17)O(NO^(–)_(3))样品的作用可能不可忽略,该作用有待结合大气化学模型进一步探索。

Atmospheric nitrate(particulate nitrate+gaseous HNO_(3))plays an essential role in the biogeochemical cycle of nitrogen in the Arctic region.However,there are poor studies on the formation mechanisms of atmospheric nitrate over the Arctic Ocean,which limits our understanding of atmospheric chemical processes related to nitrogen oxides(NO_(X)=NO+NO_(2))and nitrate in this area.As part of 5th Chinese National Arctic Research Expedition(CHINARE)in the summer of 2012,aerosol filter samples were collected and nitrogen and oxygen isotopes of nitrate(δ^(15)N,δ^(17)O andδ^(18)O)in the filter samples in 62.3°~74.7°N were analyzed to trace the formation pathways of atmospheric nitrate over the Arctic Ocean.The observed dailyΔ^(17)O(NO^(–)_(3))(=δ^(17)O–0.52δ^(18)O)varied from 21.7‰to 28.8‰with the mean of(25.4±2.7)‰andδ^(15)N(NO^(–)_(3))ranged from-7.5‰to 0.8‰with the mean of(-4.2±3.0)‰.Generally speaking,Δ^(17)O(NO^(–)_(3))showed a opposite trend with the sampling latitude,and a similar trend with the nighttime hours and O_(3)concentration.Whileδ^(15)N(NO^(–)_(3))showed the opposite trend with air temperature.Chemical kinetics calculations show that the variation ofΔ^(17)O(NO^(–)_(3))may be mainly determined by the role of different pathways in nitrate production rather than the relative importance of O_(3)and XO(X=Br,Cl and I)in NO_(X)cycling,the latter was estimated to be 0.81~0.90 with the mean of 0.85±0.03.Further analysis based onΔ^(17)O(NO^(–)_(3))showed that NO_(2)+OH pathway dominated nitrate production for samples with lowΔ^(17)O(NO^(–)_(3))(=21.7‰~24.5‰,66.2°~74.7°N),with the mean possible contribution of 68%~81%.For samples with relatively highΔ^(17)O(NO^(–)_(3))(=27.5‰~28.8‰,62.3°~69.9°N),the together role of NO_(3)+HC/DMS,NO_(3)+H_(2)O(aq)and XNO_(3)+H_(2)O(aq)are the most important,with the mean possible contribution of 35%~50%.Combined with the analysis of BrO column concentrations,it was found that the role of XNO_(3)+H_(2)O(aq)in nitrate production cannot be ignored for highΔ^(17)O(NO^(–)_(3))samples(e.g.,Δ^(17)O=28.8‰),the role of which needs to be further explored with the combination of atmospheric chemistry model in future studies.